Experimental Investigation of the Low-Voltage Arc in Noble Gases

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Abstract
An experimental investigation of the low‐voltage arc mode of the hot‐cathode discharge has been carried out in a diode utilizing planar electrode geometry. The investigation consisted predominantly of Langmuir‐probe measurements of the discharge in a neon atmosphere. Measurements were also obtained in argon, xenon, and hydrogen and hydrogen‐neon and argon‐neon mixtures. The probes were of planar, guard‐ringed geometry. The volt‐ampere characteristic of the hot‐cathode discharge in the Torr range of gas pressure was investigated to further define the low‐voltage arc. For ample electron emission at the cathode there are two stable high‐current, low‐voltage discharge modes that occur in the noble gases; these are termed the low‐voltage arc and the ball‐of‐fire modes of the hot‐cathode discharge. No low‐voltage discharge modes were obtained in hydrogen. The steady‐state characteristics obtained for the low‐voltage arc indicated peak plasma potentials of approximately 14, 6, and 4.5 V for neon, argon, and xenon, respectively, considerably lower than the first excitation level of the respective gases. There is a minimum value of the product of pressure and electrode spacing for the existence of the low‐voltage arc; this condition also yields a minimum value of discharge voltage. The discharge voltage and peak electron densities increase with increasing pressure‐ spacing product, while the peak plasma potential decreases gradually. Measurements on the discharge in neon atmospheres containing small admixtures of hydrogen indicated that cumulative ionization is important for the generation of the low‐voltage arc. It was not possible to detect in the measurements on pure noble gas experimental atmospheres any effects due to the presence of impurities. The study of the Penning impurity effect in the low‐voltage arc was limited, but showed that mixtures of less than 0.06% argon in neon do affect the discharge.

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